Occupant presence detection is frequently used in connection with air bags and other pyrotechnically deployed restraints as a means of determining if the restraints should be deployed in the event of sufficiently severe crash. Although various ways of detecting occupant presence have been devised, perhaps the most simple and cost-effective way involves installing a fluid-filled bladder in or under the bottom foam cushion of the vehicle seat and measuring the fluid pressure in the bladder with a pressure sensor. Exemplary systems of this type are disclosed, for example, in the U.S. Pat. Nos. 5,987,370 and 6,246,936 to Murphy et al., and the U.S. Pat. Nos. 6,101,436 and 6,490,936 to Fortune et al., all of which are assigned to Delphi Technologies, Inc., and incorporated herein by reference. Alternatively, the cost of the pressure sensor can be avoided by providing conductor plates on or near the upper and lower surfaces of the bladder, and detecting changes in capacitance between the plates when the thickness of the bladder changes due to the presence of occupant weight. However, the changes in capacitance tend to be relatively small, and a significant calibration effort may be required even in applications where the system merely has to detect occupant presence (as opposed to occupant weight, or weight distribution). Accordingly, what is needed is a cost-effective and easily calibrated capacitive occupant presence sensor.